Internal API changes to decouple redis from Shell

What does this MR do?

• Add a new field to /internal/check to signal redis being available
• Add a new endpoint, /internal/pre_receive to do all Shell pre-receive hook operations (for now, just increasing the reference counter).

Are there points in the code the reviewer needs to double check?

No

Why was this MR needed?

We want to decouple Shell from redis to be able to not have redis running on the NFS hosts. See https://gitlab.com/gitlab-org/gitaly/issues/467

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitaly/issues/520